1. Field of the Invention
The present invention relates to an image processing method of performing a process for converting image data (pixel value data) representative of pixel values of a plurality of pixels constituting an image into image data (halftone dot data) representative of dot patterns of halftone dots for a halftone dot image output, and an image processing apparatus for performing such a processing.
2. Description of the Related Art
Hitherto, there are used a printing machine and some color printers to form a halftone dot image in accordance with the halftone dot data.
To produce the halftone dot data in accordance with which a halftone dot image is formed, for example, image data (pixel value data) representative of pixel values of a plurality of pixels constituting an image is converted into image data (halftone dot data) representative of dot patterns of halftone dots in such a manner that when a halftone pattern comprising an array of thresholds is superimposed on the image, the pixel values of the pixels on the image are compared with the thresholds of the halftone pattern, and the pixel values of the pixels on the image are converted into multi-values (typically binary values).
Hereinafter, there will be described more in detail a process of producing of the halftone dot data referring to an example in which a printing machine is used to output a halftone dot image.
FIG. 1 is a block diagram showing an example of a printing system.
A color scanner 10 reads an original image to generate color separation image data for four colors of CMYK representative of the original image thus read. The image data for four colors of CMYK is fed to a workstation 20. In the workstation 20, an operator performs an electronic page make-up in accordance with an input image. When the electronic page make-up is completed, image data (pixel value data) representative of the image after the page make-up is generated, and the pixel value data is converted into halftone dot data for printing in a manner as will be described later. The halftone dot data is fed to a film printer 30 to produce a printing film original plate consisting of plates for CMYK associated with the entered halftone dot data.
A printing plate is produced from the printing film original plate, and then mounted on a printing machine 40. Ink is applied to the printing plate mounted on the printing machine 40. The ink thus applied is transferred to a printing paper so that a halftone dot image 41 is formed on the printing paper.
A conversion of the pixel value data into the halftone dot data in the workstation 20 is performed as follows.
FIGS. 2(A)–2(C) are explanatory views useful for understanding a conversion method of converting the pixel value data into the halftone dot data.
FIG. 2(A) is an illustration showing an example of a halftone pattern in which thresholds are arranged on a two-dimensional basis. In this halftone pattern, there are arranged the thresholds of values 1 to 25 as shown in FIG. 2(A).
FIG. 2(B) is an illustration showing part of image data (pixel value data) before the conversion, wherein there is shown a uniform image in which all pixels have pixel value 14.
Here, the image of FIG. 2(B) is partitioned into domains each having the same area as the halftone pattern of FIG. 2(A), and the halftone pattern of FIG. 2(A) is superimposed on each domain, so that the pixel value of each pixel on the image is compared with the associated threshold on the halftone pattern. When the pixel value is smaller than the threshold, the pixel value is converted into the value ‘0’, and when the pixel value is larger than the threshold, the pixel value is converted into the value ‘1’, and as a result, there is produced a binary image as shown in FIG. 2(C). Here, ink is applied to pixels of ‘1’ of FIG. 2(C) (a pattern, to which the ink is applied, is referred to as a dot pattern), while no ink is applied to pixels of ‘0’ of FIG. 2(C). That is, in FIG. 2(C), there is formed a dot pattern of a halftone dot on the same size of the halftone pattern of FIG. 2(A).
As seen from the arrangement of thresholds constituting the halftone pattern of FIG. 2(A), when pixel values of pixels constituting the image shown in FIG. 2(B) are small, the dot pattern for each halftone dot is of small area. As the pixel values of the pixels are larger, the area of the dot pattern for each halftone dot is expanded and thereby increasing density of the halftone dot image. As the pixel values of pixels are further larger, the dot patterns of the adjacent halftone dot are coupled with one another, and finally, there is formed a so-called solid image in which ink is applied to all the pixels of the whole halftone dots.
Incidentally, the halftone pattern shown in FIG. 2(A) forms a halftone dot in its entirety. The halftone pattern corresponding to the one halftone dot, or as shown in FIG. 3, when a halftone pattern corresponds to a plurality of halftone dots, a portion corresponding to a halftone dot of the whole halftone pattern, is referred to as a dot cell.
FIG. 3 is a view showing an example of a halftone pattern.
In the halftone pattern shown in FIG. 3, a plurality of dots are diagonally arranged. The halftone pattern is scanned vertically and horizontally in its entirety in such a manner that the halftone pattern is sequentially superimposed on areas of an image, so that a halftone dot image, in which halftone dots are diagonally arranged, is formed. The reason why the halftone dots are diagonally arranged is that occurrence of moirèis prevented by means of changing an angle of the arrangement for each color ink.
The halftone pattern is arranged by a combination of a plurality of dot cells. The halftone pattern, in which the plurality of dot cells are arranged, is referred to as a super-cell. The adoption of the super-cell makes it possible to perform a high speed scan.
Thus, the workstation 20 shown in FIG. 1 converts image data (pixel value data) representative of pixel values of a plurality of pixels constituting an image into image data (halftone dot data) representative of dot patterns of halftone dots. When image density represented by an area of the dot pattern on image data (halftone dot data) after conversion is not coincident with density of a halftone dot image obtained by performing actually printing in accordance with the halftone dot data. For example, when pixel values of a plurality of pixels constituting an image are sequentially varied from a small value (corresponding to low density) to a large value (corresponding to high density), the image density, which is computed from the dot pattern on the halftone dot data, is continuously varied; nevertheless, when printing is performed actually in accordance with the halftone dot data, the image density is varied discontinuously on the printed halftone dot image (such a discontinuous variation of image density is referred to as a tone jump). In some printed image, the discontinuity in density is visually recognized, and thus this brings about a problem that an image quality of the printed image is lowered.